MCT-MC
4.3
FDC MCT-MC
- 146 -
Appendix
A.1.5 Protocol
Protocol describes how to initiate an exchange. It also prevents two machines from attempting to send data at
the same time. There are a number of different data communications protocols, just as there are different
human cultural protocols that vary according to the situation.
The protocol portion of the MC communications is very important, because it provides a quality of
communication that others often don’t have. Protocol-driven communications are more accurate because they
are less prone to both operator and noise errors. Protocol maintains system integrity by requiring a response
to each message. It’s like registered mail — you know that your letter has been received because the post
office sends you a signed receipt.
In MC data communications, a dialog will continue successfully as long as the messages are in the correct form
and responses are returned to the protocol leader. If the operator enters an incorrect message, or interference
comes on to the data line, there will be no response. In that case the master must retransmit the message or
go to a recovery procedure. If an operator continues to enter an incorrect message or interference continues
on the data line, the system will halt until the problem is resolved. The MC uses Modbus RTU as the protocol
of choice. Modbus RTU enables a PC to read and write directly to registers containing the MC parameters.
With it, you can read all of the controller’s parameters with a single read command.
Modbus Remote Terminal Unit (RTU)
Gould Modicon, now called AEG Schneider, created this protocol for process control systems called "Modbus".
It has the advantage over other protocols of being extremely reliable in exchanging information. This protocol
works on the principle of packet exchanges. The packet contains the address of the controller to receive the
information, a command field that says what is to be done with the information and several fields of data. The
last item sent in the packet is a field to ensure the data is received intact. This is called a cyclic redundancy
check-sum.
See the following example for information on how to generate this value. All information is exchanged in hex
numbers. The MC only supports the binary version of Modbus, referenced as RTU. The ASCII version is less
efficient and is not supported. Therefore, you must be certain to format all data in hexadecimal.
The CRC (Cyclical Redundancy Checksum) is calculated by the following steps:
1. Load a 16-bit register (called CRC register) with 0xFFFF
2. Exclusive OR the first 8-bit byte of the command message with the low order byte of the 16-bit CRC register,
putting the result in the CRC register.
3. Shift the CRC register one bit to the right with MSB zero filling. Extract and examine the LSB.
4. If the LSB of the CRC register is zero, repeat step 3, else Exclusive OR the CRC register with the polynomial
value 0xA001.
5. Repeat steps 3 and 4 until eight shifts have been performed. When this is done, a complete 8-bit byte will
have been processed.
6. Repeat steps 2 through 5 for the next 8-bit byte of the command message. Continue doing this until all
bytes of the command message have been processed. The final contents of the CRC register is the CRC
value.
When transmitting the CRC value in the message, the upper and lower bytes of the CRC value must be
swapped, i.e. the lower order byte will be transmitted first.
Summary of Contents for MCT-MC 4.3
Page 1: ...MCT MC 4 3 User Manual MCT MC 4 3 User Manual Rev A February 2018...
Page 176: ...MCT MC4 3 FDC MCT MC 176 Appendix...
Page 177: ...MCT MC4 3 Appendix 177 FDC MCT MC Open Frame Power Supply Specifications...
Page 178: ...MCT MC4 3 FDC MCT MC 178 Appendix...
Page 179: ...MCT MC4 3 Appendix 179 FDC MCT MC A 6 Power On Delay Relay Specifications GE1A C10HA110...
Page 180: ...MCT MC4 3 FDC MCT MC 180 Appendix...
